Custom osCommerce PHP script for checkout delivery option.

I would like to give my customers the option of paying a surcharge on delivery, to enable me to collect, repair and return the product at my cost within the product’s warranty period. The customer must accept or decline the offer during checkout, so that the customer is aware that they will be liable for the cost of returning goods if they do not accept. The surcharge must be a percentage (configurable in Admin) of the shipping cost/option selected by the customer. The surcharge must be applicable per product in the customer’s basket. The customer must be able to pick which products they want to pay the surcharge on.